1. Field of the Invention
The invention relates to ultrasonic distance sensors based on the pulse-echo process with increased measurement certainty and improved suppression of interfering echo signals. Significant areas of application are non-contact distance measurement for the positioning of workpieces, collision protection or filling-level metrology.
2. Description of the Prior Art
Ultrasonic sensors are known which determine the distance between the sensor and a sound-reflecting object by measuring the transit time of a sound signal from the sensor to the object and back. In this case, the echo is usually detected in that the exceeding of a prescribed threshold value in the received signal is evaluated. This process for distance measurement usually evaluates the transit time of the first detected echo. Any possibly following echoes from other objects situated within the detection range of the sensor are not, in contrast, further processed. By means of time-window control according to Magori, V.; Walker, H.: Ultrasonic presence Sensors with wide range and high local resolution. IEEE Trans. Ultrasonics, Feroelectrics and Frequency Control, UFFC-34, No. 2, Mar. 1987, p. 202-211, the permissible detection range for echo signals can in this case be varied in the desired manner. In this manner, echoes from objects at differing distances from the sensor can also be detected, in that the evaluation time window is cyclically displaced over the measurement range; in this case, the resolution of the individual echoes and the total duration of measurement increase as the length of the time window decreases.
Processes for the processing of ultrasonic echo signals are moreover known in which the received signal is digitally sampled and stored in a memory; in this case, the received signal can also be the demodulated envelope curve of the echoes (European Application 0 459 336). The signal processing takes place following the recording of the received signal by extraction of the echoes by means of a suitable process, e.g. matched filter+threshold value detection. In this manner, all echoes occurring within one measurement can be detected.
Further, in the process which is described in Advances in Instrumentation and Control, Vol. 46, part 2, 1991, Research Triangle Park, NC, US, Duncan: "Ultrasonics in Solids Level Measurements", pages 1355-1366, the emitted and subsequently received ultrasonic signal is digitized by means of a microprocessor and stored as an envelope curve. The process relates to the measurement of the filling level of a container., To determine the useful echo, the echo profile of the empty container is compared with the echo profile of the filled container. Furthermore, it is possible to recognize the useful echo in that a priori knowledge of the nature of the filling material situated in the container and the echo characteristic thereof are used for the recognition of the useful echo.
Furthermore, processes for the suppression of undesired echoes contained in the received signal, for example due to interfering objects which, in addition to the measured object, are situated within the detection range of the sensor, are known. If the interfering objects are spatially fixed and at the same time the range of movement of the measured object is restricted, then an adequate suppression of interfering echoes can be achieved by appropriate selection of the evaluation time window.
Furthermore, it is known that interfering object echoes can be suppressed in that, in a learning phase in which the measured object is not situated within the pickup range of the sensor, in the first instance all interfering object echoes are detected and filed in a memory (German OS 33 37 690). During the measurement operation, the currently detected echoes are compared with the learned echoes. In the event of an adequate concordance, the echo is classified as an interfering object echo and appropriately suppressed, while the remaining echoes are associated with measured objects.
In German OS 33 37 690 and European Application 0 459 336, processes are described which mask out interfering echoes caused by multiple reflections between the sensor and an object in that the maximum transit time to be evaluated is limited, so that echoes occurring outside this transit time are disregarded. In the case of the solution presented in European Application 0 459 336, the echo amplitude can additionally also be evaluated as criterion for the multiple echo suppression. However, these processes are in general unsuitable for measurement situations involving a number of objects in the pickup range of the sensor.
Furthermore, processes are known for the suppression of interfering echoes in the basis of plausibility checks (German OS 38 20 103 and German OS 38 21 577). Since the extent which the measurement situation can change is limited due to the finite speed of movement of objects, echoes are evaluated only when their time position and amplitude are sufficiently plausible on the basis of their extent of deviation from previous measurement situations. In this manner, interfering signals which occur stochastically, in particular can be reliably suppressed.
It is common to all above known processes for the evaluation of echo signals in the case of ultrasonic distance sensors that an object is associated with each echo, which is detected in the received signal and which is not a stochastic interfering signal, within the maximum transit time to be evaluated; in this case, the distance from the sensor is obtained from the sound transit time of the echo. A disadvantage of these known processes is that, on this basis, echoes which arise for example as a result of multiple reflections between the sensor and an individual measured object and which do not lie outside the maximum transit time to be evaluated are thus also associated, erroneously, with further, actually non-existent objects. This may lead to very great errors in the assessment of measurement situations, especially when measured objects are situated at a short distance from the sensor.
The sound signal may be reflected repeatedly between the acoustic transducer and the objects which are situated within the pickup range of the sensor. As a function of the distance between the object and the sensor, the object reflectivity and the geometry of the acoustic transducer, as well as of the propagation attenuation, these multiple echoes decay more or less rapidly. When using a planar transducer surface or reflector surface and in the case of a short distance between the transducer and the object, the decay time constant of the multiple echoes is in the order of magnitude of the single sound transit time. The latter is obtained from the path from the sensor to the object and back. As a result, a plurality of echoes of the same object are detected in the received signal. Additional interfering echoes may occur where a plurality of objects are situated within the pickup range of the sensor. This is caused by reflection paths between the individual objects or multiple reflections on various objects.
In the case of all known processes for echo-signal processing, the problem exists that the interfering echoes arising as a result of multiple reflections are not distinguished from the direct object echoes; this leads to erroneous measurements in the case of many situations occurring in practice.